A new stacking variant of Na2Pt(OH)6

Article abstract of DOI:10.1515/znb-2018-0125

A new stacking variant of sodium hexa-hydroxo platinate(IV), Na₂Pt(OH)₆, was synthesized and its structure elucidated through X-ray diffraction. The new polymorph was prepared by direct reaction of PtO₂ with an excess of NaOH solution applying elevated oxygen pressure at 300°C. The structure consists of layers of edge sharing Pt(OH)₆ and Na(OH)₆ octahedra. These layers are separated by an edge-to-edge distance of ~2.4 ?. The packing of the hydroxide ions corresponds to the hcp sequence, the title compound thus may be regarded a cation ordered variant of the Brucite structure type. During heating above T~300°C all constitutional water is released, and anhydrous Na₂PtO₃ remains as the solid residue.

Full text of DOI:10.1515/znb-2018-0125

Z. Naturforsch. 2018; aop
Gohil S. Thakur, Hans Reuter, Claudia Felser and Martin Jansen*
A new stacking variant of Na2Pt(OH)6
https://doi.org/10.1515/znb-2018-0125
parameters, and it is of particular importance to comply
the technical approach with the chemical properties of
the constituents of the targeted compound. Fully depro-
tonated oxides can be obtained from dilute alkaline solu-
tions, in particular if the metals involved form less soluble
hydroxides, or do not form hydroxides at all, which com-
monly holds true for noble metals. Ag2PbO2 represents
such an example; it can be easily obtained phase pure and
as large crystals from ammoniated solutions upon addi-
tion of caustic soda lye [5].
When exploring the ternary system Ag/Pt/O at
elevated oxygen pressure and under alkaline (NaOH)
hydrothermal conditions, in the present work surprisingly
– along with a desired deprotonated silver oxo-platinate –
a sodium platinum(IV) hydroxo complex, Na2Pt(OH)6, had
also crystallized. Its composition was confirmed by X-ray
Received June 25, 2018; accepted August 30, 2018
Abstract: A new stacking variant of sodium hexa-hydroxo
platinate(IV), Na2Pt(OH)6, was synthesized and its struc-
ture elucidated through X-ray diffraction. The new poly-
morph was prepared by direct reaction of PtO2 with an
excess of NaOH solution applying elevated oxygen pres-
sure at 300°C. The structure consists of layers of edge
sharing Pt(OH)6 and Na(OH)6 octahedra. These layers are
separated by an edge-to-edge distance of ~2.4 Å. The pack-
ing of the hydroxide ions corresponds to the hcp sequence,
the title compound thus may be regarded a cation ordered
variant of the Brucite structure type. During heating above
Tꢀ~ꢀ300°C all constitutional water is released, and anhy-
drous Na2PtO3 remains as the solid residue.
Keywords: crystal structure determination; hydrothermal crystal structure analyses, and the hydrothermal synthe-
synthesis; platinate(IV).
sis conditions were eventually optimized, starting from
PtO2 and NaOH, exclusively.
Na2Pt(OH)6 as such was first reported in 1905 [6], and
its crystal structure was determined from powder X-ray [7]
and neutron diffraction data (collected on a deuterated
sample) [8]. The structure analysis based on neutron data
enabled to localize the deuterium atoms and to assign the
correct space group, which was confirmed later by X-ray
single crystal studies [9].
Under the synthesis conditions applied in this work,
reproducibly a new polymorph has been obtained, which is
a particular stacking variant of the known polymorph [7–9].
Reducing the title compound to its binary aristotype, AX2,
it becomes evident that the basic structural organization of
both the modifications corresponds to the CdI2 type of struc-
ture, featuring different cation ordering schemes in the dif-
ferent polymorphs. Due to the higher temperature applied
during synthesis of the title compound, compared to the
earlier reported, we regard it a high temperature modifica-
tion and thus assign the prefixes according to the conven-
tions, α-Na2Pt(OH)6 [7–9] and β-Na2Pt(OH)6, this work.
Dedicated to: Professor Bernt Krebs on the occasion of his 80th
birthday.
1 Introduction
Hydrothermal synthesis is among the most prolific and
versatile methods in use for the preparation of solid
oxide materials [1–3]. Going beyond the original defini-
tion of the term, this approach does no longer exclusively
imply a supercritical state of the solvent [1], and the field
of applicability has been widened significantly by taking
advantage of tuning the viscosity and the hydroxide activ-
ity between concentrated to diluted aqueous solutions
of hydroxides and molten hydroxides. For cases where
water is added in order to adjust the solution properties
of a melt, the term “hydroflux” has come into use [4].
Obviously, such procedures of preparing solid materials
require to control a considerable number of experimental
*Corresponding author: Martin Jansen, Max-Planck-Institut fur
Chemische Physik fester Stoffe, Nothnitzer Straße 40, Dresden
01187, Germany; and Max-Planck-Institut fur Festkorperforschung,
Heisenbergstraße 1, 70569 Stuttgart, Germany,
2 Experimental
e-mail: m.jansen@fkf.mpg.de
2.1 Crystal growth
Gohil S. Thakur and Claudia Felser: Max-Planck-Institut fur Chemische
Physik fester Stoffe, Nothnitzer Straße 40, Dresden 01187, Germany
Hans Reuter: Institut fur Chemie neuer Materialien, Universität
Osnabruck, Barbarastraße 7, 49076 Osnabruck, Germany
Millimeter sized single crystals of Na2Pt(OH)6 were grown
by heating finely powdered PtO2 and 1–2 mL of 3–5 m NaOH
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ꢀꢀꢀꢁ
2ꢀ ꢀG.S. Thakur et al.: A new stacking variant of Na2Pt(OH)6
solution at 300°C under high oxygen pressure. Commer- arrangements with the exception of disordered Na atoms
cially purchased powder of PtO2 was used without further in the centrosymmetric one arising from a crystallo-
purification (Sigma-Aldrich 99.9%). 0.189 g of PtO2 fine graphic mirror plane through the Pt atoms. Finally, the
powder was placed in a gold crucible, flame sealed at one absolute structure parameter of 0.016(11) convinced us to
end, and 1.5 mL of 5 m NaOH was added on top of it. The use the non-centrosymmetric structure model. Structure
crucible was mechanically crimped and placed inside plots were drawn using Diamond [16].
a 20 mL stainless steel autoclave [10]. 11.2 mL of liquid
Both hydrogen atoms were localized in a difference
oxygen was condensed into it. The said amount of oxygen Fourier synthesis. Their positions were initially refined
generates about 2500 bars of pressure at the given tem- while constraining the O–H distances to 0.96 Å, and
perature. After the complete condensation of oxygen, the finally they were allowed to ride on the corresponding
top of the autoclave vessel was sealed by a copper bullet oxygen atom. During all final calculations, the lattice
and fastened to make it gas-tight. The assembly was then parameters as obtained from X-ray powder pattern refine-
placed in a vertical programmable furnace at Tꢀ=ꢀ300°C ment were used.
for 4 days after which it was slowly cooled to room tem-
Crystallographic data, details of the data collec-
perature over 24 h. Crystals were separated by means of tion and of the structure refinement are given in Table 1,
vacuum filtrations and subsequently cleaned from any atomic coordinates, anisotropic and equivalent isotropic
residues by ultra-sonication in alcohol.
displacement parameters in Table 2, and bond lengths in
Table 3.
Further details of the crystal structure investigation
may be obtained from FIZ Karlsruhe, 76344 Eggenstein-
Leopoldshafen, Germany (fax: (+49)7247-808-666; e-mail:
2.2 Powder X-ray diffraction
Laboratory powder X-ray diffraction (PXRD) studies at RT crysdata(at)fiz-karlsruhe(dot)de, on quoting the deposi-
were performed with a HUBER G670 imaging plate Guinier tion number CSD-434726.
camera with CuKα1 radiation (λꢀ=ꢀ1.5406 Å), in a 2θ range
of 5ꢀ−ꢀ85°. Profile refinement, a le Bail fit, was carried out
with the program Topas (Bruker AXS; version 4.2.0.2) [11].
Table 1:ꢀCrystal data and numbers pertinent to data collection and
The refined parameters were scale factor, zero point of θ,
sample displacement (mm), and background as a Cheby-
chev polynomial of 40th degree and 1/x function, crystal-
lite size, microstrain, and cell constants.
structure refinement of β-Na2Pt(OH)6.
Empricial formula
Na2O6H6Pt
Mr
343.11
Cryst. size, mm3
Temperature, K
Crystal system
Space group
a, Å
0.163ꢁ×ꢁ0.111ꢁ×ꢁ0.083
296(2)
Trigonal
2.3 Single-crystal X-ray diffraction
P31c (no. 159)
5.8075(1)
Ambient-temperature single-crystal diffraction data of an
irregularly shaped crystal fragment of appropriate size
were collected on a Bruker Kappa APEX II CCD-based
4-circle X-ray diffractometer using graphite-monochroma-
tized MoKα radiation (λꢀ=ꢀ0.71073 Å) of a fine focus molyb-
denum-target X-ray tube operating at 50 kV and 30 mA.
Collection and reduction of data were carried out with the
Bruker Suite software package [12]. Intensities were cor-
rected for absorption effects semi-empirically from equiv-
alents via Sadabs [13]. The structure was solved by Direct
Methods and refined by full-matrix least-squares fitting
on F2 with the shelxl software package [14]. Systematic
absence conditions hh2hl only present for lꢀ=ꢀ2n are con-
b, Å
9.3931(2)
V, Å3
274.36(1)
Z
2
D
calcd, g cm−3
4.153
μ(MoKα), mm−1
F(000), e
hkl range
θ range, deg
25.7
308
−8ꢁ≤ꢁhꢁ≤ꢁ8, −8ꢁ≤ꢁkꢁ≤ꢁ8, −14ꢁ≤ꢁlꢁ≤ꢁ14
4.052–31.982
30ꢁ961/647
Refl. collected/unique
Rint
Completeness to θꢁ=ꢁ31.98°, %
Absorption correction
Refinement method
Data/param. refined
R1/wR2 [Iꢁ>ꢁσ(I)]
0.0467
100
Semi-empirical from equivalents
Full-matrix least-squares on F2
647/29
0.0190/0.0486
0.0197/0.0490
0.0019(6)
̅
sistent with the centrosymmetric space group P31c (No. 163 R1/wR2 (all reflexions)
Extinction coefficient
[15]) and the non-centrosymmetric one P31c (No. 159 [15]).
x(Flack)
0.016(11)
3.74/−ꢁ1.59
Consequently, structure refinements were performed in
Largest diff. peak/hole, e Å−3
both space groups, giving rise to very similar cation-anion
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ꢂ3
G.S. Thakur et al.: A new stacking variant of Na2Pt(OH)6ꢂ
Table 2:ꢀAtomic coordinates, anisotropic and equivalent isotropic displacement parameters (Å2ꢁ×ꢁ10−3) for β-Na2Pt(OH)6.
Atom
Wyckoff site
x
y
z
U11
U22
U33
U12
U13
U23
Ueq/iso
Pt(1)
Na(1)
Na(2)
O(1)
O(2)
H(1)
2b
2b
2a
6c
6c
6c
6c
2/3
1/3
1/3
2/3
1/2
0.519(1)
0.4633(7)
0.378(1)
0.621(1)
0.2046
6(1)
9(1)
19(2)
6(3)
11(4)
6(1)
9(1)
19(2)
11(4)
9(4)
6(1)
24(8)
16(3)
13(3)
10(4)
0
0
0
0
3(1)
5(1)
9(1)
7(3)
5(3)
6(1)
14(2)
18(1)
9(2)
10(2)
40
0
0
0
0
0.637(2)
0.940(3)
0.5501
0.7986
0.599(3)
0.632(3)
0.4685
0.6098
1(3)
−4(3)
−1(3)
−3(3)
H(2)
0.6834
40
The anisotropic displacement factor exponent takes the form: −2π2(U11h2a*2ꢁ+ꢁU22k2b*2ꢁ+ꢁU33l2c*2ꢁ+ꢁ2 U12hka*b*ꢁ+ꢁ2 U13hla*c*ꢁ+ꢁ2 U23klb*c*), Ueq
is defined as one third of the trace of the orthogonalized Uij tensor.
Table 3:ꢀCharacteristic bond lengths (Å) for β-Na2Pt(OH)6.
hydrothermal conditions, while maintaining elevated
oxygen pressure (~2500 bars). Pale yellow, coarse crystal-
line platelets with hexagonal habit and dimensions up to
Atoms
Multiplicity
Bond length
Pt1‒O1
Pt1‒O2
Na1‒O1
Na1‒O2
Na2‒O1
Na2‒O2
3ꢁ×ꢁ
3ꢁ×ꢁ
3ꢁ×ꢁ
3ꢁ×ꢁ
3ꢁ×ꢁ
3ꢁ×ꢁ
1.998(13) 5 mm across, were obtained after vacuum filtration, see
2.018(13)
Fig. 1. The filtrate appeared slightly brownish in color indi-
cating that some unreacted ultrafine PtO2 remained in the
2.392(14)
2.393(13)
2.367(11)
2.477(13)
2.4 Thermal analysis
Thermogravimetric (TGA) analyses of β-Na2Pt(OH)6 were
carried out on a Netzsch STA 449 C analyzer. Approxi-
mately 35 mg of powdered sample was placed in a corun-
dum crucible, which was heated and cooled back at a rate
of 5 K min−1 in the temperature range of 25–1000°C under
dynamic argon flow.
Fig. 2:ꢀle Bail fit to the laboratory powder X-ray diffraction data
(λꢁ=ꢁ1.5406 Å) collected at room temperature for β-Na2Pt(OH)6. The
black and red curves correspond to experimental and calculated
data, respectively; the blue line is the difference between the two,
and the vertical bars correspond to the calculated Bragg peak
positions.
3 Results and discussion
A new polymorph of Na2Pt(OH)6 (the β-phase) has been
obtained from PtO2 and NaOH solution (5 m) under
Fig. 1:ꢀPhotographs of the as-grown crystals of Na2Pt(OH)6 on a millimeter grid.
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ꢀꢀꢀꢁ
4ꢀ ꢀG.S. Thakur et al.: A new stacking variant of Na2Pt(OH)6
solution. Only large crystalline aggregates were obtained diffraction data allowed to identify the different cation
in bulk quantitative yield and no polycrystalline product ordering which indeed corresponds to a new stacking
was observed.
A powder X-ray diffractogram of crushed crystals
variant of Na2Pt(OH)6.
The title compound has a layer structure, where the
shows a pattern different from that of the reported hydroxide anions follow the stacking sequence of a hex-
Na2Pt(OH)6 (α-phase). From single crystal diffrac- agonal closest packing and the cations occupy the octa-
tion and indexing of the powder pattern a trigonal hedral voids in every second spacing, see Fig. 3. Thus,
crystal system was identified, with a le Bail fit to the the general structural organization corresponds to the
PXRD resulting in lattice constants of aꢀ=ꢀ5.8075(1) and CdI2 type of structure, or to the chemically more closely
cꢀ=ꢀ9.3931(2) Å, see Fig. 2. Clearly, the c parameter is related Brucite structure, Mg(OH)2. The cations Na and Pt
twice that of the α-phase, indicating a different stack- form a fully ordered distribution, where according to the
ing sequence in the present compound. Single crystal ratio of 2:1, the Pt(OH)6 octahedra are “isolated” and share
Fig. 3:ꢀProjections of the crystal structure of β-Na2[Pt(OH)]6 along [001], left, and [010], right. Pt(OH)6 octahedra in gray, Na(OH)6 octahedra
in green.
Fig. 4:ꢀCrystal structure of Na2Pt(OH)6, details of the layer constitution (side view, same scale) showing the structural similarity, and
emphasizing the violation of the crystallographic mirror plane, in the α- (above) and β- (below) modification of Na2[Pt(OH)]6. In the β-
modification all non-H atoms are drawn as displacement ellipsoids on a 90% level to relate the size of the atoms in the ball-and-stick model
of the α-modification (drawn according to [9]). Numbers are indicating distances (Å) of the atoms from the planes defined by the Pt atoms.
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G.S. Thakur et al.: A new stacking variant of Na2Pt(OH)6ꢂ
Fig. 5:ꢀStacking sequence of occupied octahedral holes [Naꢁ=ꢁgreen, Ptꢁ=ꢁgray)] in the [AB]/hcp packing of oxygen atoms [red] in the α- (left,
according to [8, 9]) and β- (right) modification of Na2Pt(OH)6.
Fig. 6:ꢀThermal decomposition profile of β-Na2Pt(OH)6 (left) and PXRD data of the solid residue (right).
common edges with six surrounding distorted Na(OH)6 of 2.008 Å. The Pt(OH)6 octahedra are isolated from one
octahedra, see Fig. 3.
another and are laterally surrounded by the two types of
Both polymorphs contain the same slabs, as can edge sharing distorted octahedra, Na1(OH)6 and Na2(OH)6,
be seen in Fig. 4. In α-Na2Pt(OH)6 the slabs are stacked in an alternating fashion.
along the c axis keeping the same orientation, while in
Thermal decomposition of β-Na2Pt(OH)6 was monitored
β-Na2Pt(OH)6 the layers are shifted alternatingly by a via DTA/TGA under argon up to Tꢀ=ꢀ1000°C, see Fig. 6 (left).
vector of 1/3, 2/3, 0, forth and back, see Fig. 5. The c lattice The compound shows practically no weight loss up to
parameter for the latter is thus twice that of the reported Tꢀ=ꢀ280°C above which it loses constitutional water, three
form of α-Na2Pt(OH)6 [7–9] with a similar a parameter. The H2O molecules per formula unit, as inferred from a total
stacking sequence found for β-Na2Pt(OH)6 implies reduced weight loss of ~16.4%. According to weight calculation,
Pt–Pt repulsion compared to the α-polymorph, rendering the bright orange/red residue corresponds to the formula
the former electrostatically favored.
Na2PtO3, as also observed in the thermal decomposition of
The Pt atoms occupy the centers of slightly dis- α-Na2Pt(OH)6 [9]. The dip in the DTA curve at 821°C corre-
torted Pt(OH)6 octahedra with a mean Pt‒O bond length sponds to the melting of Na2PtO3. The X-ray diffractogram of
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ꢀꢀꢀꢁ
6ꢀ ꢀG.S. Thakur et al.: A new stacking variant of Na2Pt(OH)6
[3] G. Demazeau, A. Largeteau, Z. Anorg. Allg. Chem. 2015, 641,
159.
[4] D. E. Bugaris, M. D. Smith, H.-C. zur Loye, Angew. Chem. Int. Ed.
2012, 51, 3780.
this residue confirms the formation of Na2PtO3 along with a
small amount of Pt metal (Fig. 6, right).
[5] M. Jansen, M. Bortz, Z. Anorg. Allg. Chem. 1989, 579, 123.
[6] J. Bellucci, Z. Anorg. Chem. 1905, 44, 179.
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Software for Powder Diffraction Data, Bruker AXS GmbH,
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4 Conclusions
A
new stacking variant of sodium hexa-hydroxo
platinate(IV) has been obtained as coarse single crystals
by hydrothermal synthesis under oxygen pressure. The
general organization of the atoms corresponds to the CdI2,
or Mg(OH)2, type of structure where the cations Na and Pt
are fully ordered. The new β-Na2Pt(OH)6 polymorph con-
sists of Pt(OH)6 octahedra surrounded by two types of
Na(OH)6 octahedra in an edge sharing fashion.
[12] Apex 2, Suite of Crystallographic Software, Bruker AXS Inc.,
Madison, Wisconsin (USA) 2008.
[13] G. M. Sheldrick, Sadabs, Bruker AXS Inc., Madison, Wisconsin
(USA) 2008.
[14] G. M. Sheldrick, Acta Crystallogr. 2008, A34, 112.
[15] T. Hahn (Ed.), International Tables for Crystallography, 4th ed.,
Vol. A, Space-Group Symmetry, Kluwer Academic Publishers,
Dordrecht 1996.
[16] K. Brandenburg, Diamond, Crystal and Molecular Visualization,
Crystal Impact, Bonn (Germany) 2006.
References
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[2] D. R. Modeshia, R. I. Walton, Chem. Soc. Rev. 2010, 39, 4303.
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Z. Naturforsch.ꢂ
ꢂ2018 | Volume x | Issue x(b)
Graphical synopsis
Gohil S. Thakur, Hans Reuter, Claudia
Felser and Martin Jansen
A new stacking variant of Na2Pt(OH)6
https://doi.org/10.1515/znb-2018-0125
Z. Naturforsch. 2018; x(x)b: xxx–xxx
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